Research advances in pyrolysis of softwood lignin-based monomers

Abstract: Lignin is the second largest natural polymer material in lignocellulose-based biomass components, just behind cellulose and the only sustainable source to produce renewable aromatic compounds. However, lignin is always treated as the cause of serious environmental problems, as it was burned under low temperature and discharged arbitrarily without effectively utilization. Pyrolysis technology offers an effective way for fast conversion of lignin into biochar, biooil and biogas products, to realize its high-value utilization and valorization. In recent years, Guaiacyl structural units, the main component structures of softwood lignin and gramineous lignin, are widely used as model compounds for the understanding of decomposition and coking mechanisms in lignin pyrolysis, not to mention their functional groups, such as methoxy and phenolic hydroxyl groups in their structures widely exist in lignin. This review summarized the pyrolysis process, impact factors, product distribution, and catalyst deactivation mechanism, during guaiacol-based model compounds pyrolysis process. During the direct pyrolysis process of guaiacol compounds, the main products mainly including phenols and catechol compounds, and pyrolysis temperature showed a certain influence on the guaiacol compounds conversion and products distributions. Increasing the pyrolysis temperature can increase the conversion rate, while lead to produce more olefins and a small number of aromatics. Moreover, the C4 substituent functional group of guaiacol-type compounds (e.g. vanillin, vanillic acid and vanillyl alcohol) also affects the pyrolysis product distributions. In catalytic pyrolysis, most previous studies focused on the catalytic pyrolysis of guaiacol, in which aromatic hydrocarbons and phenols compounds served as the main products, Zeolites, especially HZSM-5 based catalysts dominated. The unique structure and acidic sites of zeolite-based catalysts are the main active sites for guaiacol conversion and products formation during catalytic pyrolysis process. The addition of hydrogen donors can significantly increase the deoxygenation rate of guaiacol, while, reduce the carbon deposition of the catalyst. The impact factors, such as pyrolysis temperature, Weight Hourly Space Velocity (WHSV), and guaiacol partial pressure, strongly affect the catalytic pyrolysis of guaiacol. Increasing the pyrolysis temperature can enhance the coke formation on the catalyst, and promote the production of aromatic hydrocarbons and olefins, whereas, increasing the WHSV and guaiacol partial pressure can inhibit the coke formation on the catalyst, and reduce the efficiency of deoxygenation, leading to more phenolic compounds production, and guaiacol partial pressure. The deactivation of the catalyst is mainly resulted from the loss of active sites, and the blockage of the channel caused by carbon deposition of its surface area. In the pyrolysis mechanism, the pyrolysis of guaiacol-based compounds is mainly a free radical reaction. Catechol is mainly generated through the homolytic cleavage of O-CH3 bond, when phenol is mainly produced through the demethoxylation pathways, which was promoted by the H-atom and CH3-radical. Catechol and its derivatived o-hydroxybenzoquinone are the key intermediates during the production of gas. The aromatic hydrocarbons formation during the catalytic pyrolysis is mainly through the hydrocarbon pools pathways. At first, Guaiacol participates in the pyrolysis reaction to form the intermediates, such as phenol, catechol, then exists as the intermediates to form a hydrocarbon pool inside the catalyst, and finally converts into aromatics and olefins. This critical review can be necessary to further deepen the understanding of the lignin pyrolysis process, and thereby to provide some theoretical guidance for the regulation of lignin pyrolysis products.